JET : Java Emitter Templates
Arthur J. Wanner, Jr.
Frankie Kwok
Outline
What is M2T?
What is JET?
How JET works: Models & Templates
Small Hello World Example Tutorial
JET language and syntax
PathwayMATE code generation example
What is M2T? (Model-To-Text)
Starts for a “model” – some sort of
structured document
Examples: XML, UML, EMF-based
models
During execution, model is traversed and
“templates” are executed, creating strings
(which can be used to output to stdout,
files, etc.)
The Idea of JET
Advantages of JET
Java as a powerful template language
Skeletons for customized generator classes
Arbitrary source model
(e.g. models, arrays / lists, files, …)
Arbitrary text output (e.g. text-reports, code, xml, …)
Disadvantages of JET
Bound to Eclipse
Compilation of generator classes required
Produces a .JETEmitter project for internal
purposes
If a template is changed, the generator class is
overwritten
Hard to debug
No syntax check for templates
(JET editor with syntax highlighting etc. still buggy)
Transformation logic and output are mixed up
Optimized for text output
generating models is more complicated
Hello World Example
Create a new Project in Eclipse
Convert the Project to a JET project
In Navigator, right click on the project...
New > Other... > Java Emitter Templates >
Convert Projects to JET Projects
This will create a 'templates' top-level dir
In the templates dir create a new file called
helloworld.txtjet
<%@ jet package="hello"class="HelloWorldTemplate" %>
Hello, <%=argument%>!
Hello World Example continued...
When the template file is saved it creates a new
template implementation class
The implementation class name is taken from the
class property and is located in the package specified
by the package property
<%@ jet package="hello"class="HelloWorldTemplate" %>
Hello, <%=argument%>!
Hello World Example continued...
The implementation class is to be used by your code
to output the generated text...
The generate method takes an object parameter
named 'argument' which can safely be used as a
string in your template.
package hello;
public class HelloWorld {
public static void main(String args[]) {
HelloWorldTemplate hw = new HelloWorldTemplate();
System.out.println(hw.generate("World"));
}
}
JET Language
Directives
<%@ … %>
There are two types of directives:
jet (must be in 1st line!)
parameters: package, class, imports,
startTag, endTag, skeleton, nlString
include (optional)
parameters: file, fail
<%@ jet package="hello" class="HelloWorldTemplate" %>
Hello, <%=argument%>!
JET Language
Include Directive
The include directive is used to substitute text and/or code at template
translation-time. The <%@ include file="urlSpec" %> directive inserts the text of
the specified resource into the jet template file. The included file may have JET
scripting elements which will also be processed.
The value of this attribute is the URI of the location of the file to include. This
URI can be either an absolute path or a relative path. Relative URIs are always
interpreted as relative to the folder of the template that contains the include
directive.
Example:
The following example requests the inclusion, at translation time, of a copyright
file.
<%@ include file="copyright.jet" %>
JET Language
Expressions
Arbitrary java expressions that output strings
<%= … %>
<%@ jet package="hello" class="HelloWorldTemplate" %>
Hello, <%=argument%>!
<%=new java.util.Date()%>
JET Language
Scriptlets
Arbitrary java code
<% … %>
for loops
<%@ jet package="hello" class="HelloWorldTemplate" %>
<% for (int i = 0; i < 5; i++) { %>
Hello, <%=argument%>!
<%}%>
JET Language
Predefined variables
argument
object to which the transformation is applied to
can be any object but by default it's cast to a
string
<%@ jet package="hello" imports="pack.Person" class="PersonTemplate"%>
<% Person person = (Person) argument; %>
Name = <%= person.getName(); %>
Height = <%= person.getHeight(); %>
Eye Color = <%= person.getEyeColor(); %>
JET Language
Predefined variables
stringBuffer
output of the transformation
arbitrary text can be passed to the stringBuffer
used within scriplets
<%@ jet package="hello" class="HelloWorldTemplate" %>
<%String hw = "Hello " + argument + NL; %>
<%for(int i = 0; i < 3; i++, stringBuffer.append(hw))%>
<%;%>
JMerge
JMerge is a technology developed by the Eclipse
Modeling Framework (EMF) so that code generators
that produce Java code can permit user modification
of that code, while still permitting the code generator
to be re-executed.
A contract between Code Generator and the
Consumer
The Code Generator shall only update Java elements
with an intact @generated tag.
The Code Generator may remove Java elements with
an intact @generated tag this is no longer specified
by the code generator input.
JET : Java Emitter Templates
Arthur J. Wanner, Jr.
Frankie Kwok
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